Class of charge-actuated chromogenic structures based on the oxidation and reduction of optical switchable materials in a thin-film electrochemical cell

ABSTRACT

An electrochemically-activated optical switch is provided, comprising a molecular system configured between a pair of electrodes. The molecular system includes a moiety that is oxidizable or reducible in the presence of an electric current.

TECHNICAL FIELD

The present invention relates generally to optical switching, and, moreparticularly, to improved optical switching speed, and/or electricalpower dissipation, relying on reversible (electrochemical)oxidation-reduction reactions.

BACKGROUND ART

Currently, there are a wide variety of known chromogenic materials thatcan provide optical switching in thin film form. These materials andtheir applications have been reviewed recently by C. B. Greenberg inThin Solid Films, Vol. 251, pp. 81-93 (1994); R. J. Mortimer in ChemicalSociety Reviews, Vol. 26, pp. 147-156 (1997); and S. A. Agnihotry inBulletin of Electrochemistry. Vol. 12, pp. 707-712 (1996).

Such chromogenic materials are currently being studied for severalapplications, including active darkening of sunglasses, active darkeningof windows for intelligent light and thermal management of buildings,and various types of optical displays (such as heads-up displays on theinside of windshields of automobiles or airplanes and eyeglassdisplays). Despite their long history of great promise, there are veryfew photon gating devices made from the existing classes ofelectrochromic materials. This is because most of them require anoxidation-reduction reaction that involves the transport of ions, suchas H⁺, Li⁺ or Na⁺, through some type of liquid or solid electrolyte.Finding the appropriate electrolyte is a major problem, as is the slowspeed of any device that requires transport of ions. Furthermore, suchreactions are extremely sensitive to background contamination, such asoxygen and other species, and thus degradation of the chromogenicelectrodes is a major limitation.

In fact, for photonic switching applications such as a crossbar switchrouter for a fiber optic communications network, the lack of a suitablechromogenic material has forced companies to use very differentapproaches: (a) transform the optical signal into an electronic signal,perform the switching operation, and then transform back to an opticalsignal before launching into a fiber—this is the most frequent solutionused today but it is very inefficient and the electronics have a hardtime keeping up with the data rates of the optical system; (b) use amoving-mirror array made by micro-electromechanical processing to switchoptical data packets—this has the disadvantage that extremely hightolerances are required for the device, which makes it very expensive,and (c) use ink jet technology to push bubbles into a chamber to createa mirror to deflect an optical beam—this approach again requiresprecision manufacturing and the switching time is slow.

Thus, there remains a need for an optical switch that can rapidly switchoptical signals from one path to another with low power dissipation.

DISCLOSURE OF INVENTION

In accordance with the teachings of the embodiments herein, anelectrochemically-activated optical switch is provided, comprising amolecular system configured between a pair of electrodes. The molecularsystem includes a moiety that is oxidizable or reducible in the presenceof an electric current induced by an applied voltage of the appropriatemagnitude and sign.

A primary advantage of the present invention is simplification (and thuseasier manufacturability and lower cost) of the apparatus required forswitching.

A second major advantage of the present invention is improved speed ofthe switching process, since die time scale for switching is determinedby the injection or extraction of electrons and holes directly into orfrom the reductant and oxidant, in a fashion similar to charging ordischarging a capacitor, rather than transport of an ion through a thickelectrolyte layer.

A third major advantage is that the voltage and the amount of power usedto change the color of the structure are quite low, and in fact sincethe structure is essentially a very thin battery, much of the energyrequired to store information in the system can be reclaimed later uponerasing or changing the color state of the material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation (perspective, transparent view) ofa two color (e.g., black and white) display screen construction for usein accordance with the present invention;

FIG. 1a is a detail for a colorant layer element of the display screendepicted in FIG. 1;

FIG. 2 is a schematic representation (perspective, transparent view) ofa full-color display screen construction for use in accordance with thepresent invention;

FIG. 3 is a schematic representation of a scan addressing embodiment ofa two-color display screen construction for use in accordance with thepresent invention;

FIG. 4a is a schematic drawing depicting one embodiment of acolor-switching device of the present invention in a first state; and

FIG. 4b is a drawing similar to that of FIG. 4a, but in a second, orswitched, state.

BEST MODES FOR CARRYING OUT THE INVENTION Definitions

The term “self-assembled” as used herein refers to a system thatnaturally adopts some geometric pattern because of the identity of thecomponents of the system; the system achieves at least a local minimumin its energy by adopting this configuration.

The term “singly configurable” means that a switch can change its stateonly once via an irreversible process such as an oxidation or reductionreaction; such a switch can be the basis of a programmable read-onlymemory (PROM), for example.

The term “reconfigurable” means that a switch can change its statemultiple times via a reversible process such as an oxidation orreduction; in other words, the switch can be opened and closed multipletimes, such as the memory bits in a random access memory (RAM) or acolor pixel in a display.

The term “bi-stable” as applied to a molecule means a molecule havingtwo relatively low energy states (local minima) separated by an energy(or activation) barrier. The molecule may be either irreversiblyswitched from one state to the other (singly configurable) or reversiblyswitched from one state to the other (reconfigurable). The term“multi-stable” refers to a molecule with more than two such low energystates, or local minima.

The term “micron-scale dimensions” refers to dimensions that range from1 micrometer to a few micrometers in size.

The term “sub-micron scale dimensions” refers to dimensions that rangefrom 1 micrometer down to 0.05 micrometers.

The term “nanometer scale dimensions” refers to dimensions that rangefrom 0.1 nanometers to 50 nanometers (0.05 micrometers).

Micron-scale and submicron-scale wires refer to rod or ribbon-shapedconductors or semiconductors with widths or diameters having thedimensions of 0.05 to 10 micrometers, heights that can range from a fewtens of nanometers to a micrometer, and lengths of several micrometersand longer.

“HOMO” is the common chemical acronym for “highest occupied molecularorbital”, while “LUMO” is the common chemical acronym for “lowestunoccupied molecular orbital”. HOMOs and LUMOs are responsible forelectronic conduction in molecules and the energy difference between theHOMO and LUMO and other energetically nearby molecular orbitals isresponsible for the color of the molecule.

An optical switch, in the context of the present invention, involveschanges in the electromagnetic properties of the molecules, both withinand outside that detectable by the human eye, e.g., ranging from the farinfra-red (IR) to deep ultraviolet (UV). Optical switching includeschanges in properties such as absorption, reflection, refraction,diffraction, and diffuse scattering of electromagnetic radiation.

The term “transparency” is defined within the visible spectrum to meanthat optically, light passing through the colorant is not impeded oraltered except in the region in which the colorant spectrally absorbs.For example, if the molecular colorant does not absorb in the visiblespectrum, then the colorant will appear to have water cleartransparency.

The term “omni-ambient illumination viewability” is defined herein asthe viewability under any ambient illumination condition to which theeye is responsive.

Optical Switches

Optical switches are described in greater detail in copending U.S.application Ser. No. 10/187720, filed on Jul. 1, 2002, [PD-10005747-1].Although that application is directed more generally to electric fieldswitchable colorants, the basic teachings of that application also applyto electrochemically switchable colorants. In the former case, a voltageis applied to cause the switching, while in the latter case, a currentinduced by the application of a voltage of the appropriate magnitude andsign causes the electrochemical oxidation or reduction of species. Theembodiments herein are directed to the latter approach.

A generic example taken from the above-referenced application isdepicted herein in FIG. 1, wherein a display screen 100 is shown thatincorporates at least one colorant layer 101. The colorant layer 101comprises a pixel array using electrochemically switchable,reconfigurable, dye or pigment molecules of the present invention,described in greater detail below and generically referred to as a“molecular colorant”. Each dye or pigment molecule is electrochemicallyswitchable either between an image color (e.g., black) and transparentor between two different colors (e.g., red and green).

Referring briefly to FIG. 1a, the colorant layer 101 is an addressablepixel array formed of bi-stable molecules arrayed such that a selectedset of molecules correlates to one pixel. The colorant layer 101 is athin layer coated on a background substrate 103 having the display'sintended background color (e.g., white). The substrate 103 may comprise,for example, a high dielectric pigment (e.g., titania) in a polymerbinder that provides good white color and opacity. The stratifiedcombination of colorant layer 101 and substrate 103 thus is fullyanalogous to a layer of ink on paper. In a blank mode, or erased state,each molecule is switched to its transparent orientation; the “layer ofink” is invisible. The background (e.g., white pixels) shows through inthose pixel areas where the colorant layer 101 molecules are switched tothe transparent orientation. A transparent view-through layer 105, suchas of a clear plastic or glass, is provided superjacent to thecolorant-background sandwich to provide appropriate protection. Theview-through layer 105 has a transparent electrode array 107 for pixelcolumn or row activation mounted thereto and positioned superjacent tothe colorant layer 101. The background substrate 103 has a complementaryelectrode array 109 for pixel row or column activation mounted thereto(it will be recognized by those skilled in the art that a specificimplementation of the stratification of the electrode arrays 107, 109for matrix addressing and field writing of the individual pixels mayvary in accordance with conventional electrical engineering practices).Optionally, the pixels are sandwiched by employing thin film transistor(TFT) driver technology as would be known in the art.

The present display 100 is capable of the same contrast and color ashard copy print. A molecular colorant is ideal because its size and massare infinitesimally small, allowing resolution and colorant switchingtimes that are limited only by the field writing electrodes andcircuitry. Like ink, the colorant layer 101 may develop adequate densityin a sub-micron to micron thin layer, potentially lowering the totalcharge required to switch the colorant between logic states and thusallowing the use of inexpensive drive circuitry.

Suitable reconfigurable bi-stable molecules for use in such displays aredisclosed below and claimed herein. In the main, these molecules haveoptical properties (e.g., color) that are determined by the extent oftheir π orbital electron conjugation. The optical properties, includingcolor or transparency, of the molecules change with oxidation/reductionof the molecules and remain chromatically stable. By disrupting thecontinuity of conjugation across a molecule, the molecule may be changedfrom one optical state to another, e.g., colored to transparent.Specific functional groups may be designed into the colorant that canphysically cause this disruption by rotating or otherwise distortingcertain segments of the dye or pigment molecule relative to othersegments, when the molecule is oxidized or reduced.

The colorant layer 101 is a homogeneous layer of molecules which arepreferably colored (e.g., black, cyan, magenta, or yellow) in amore-conjugated orientation and transparent in a less-conjugatedorientation. By making the abutting background substrate 103 white, thecolorant layer 101 may thereby produce high contrast black and white,and colored images. The colorant layer 101 may comprise a singleelectrochemically switchable dye or pigment or may comprise a mixture ofdifferent electrochemically switchable dyes or pigments thatcollectively produce a composite color (e.g., black). By using amolecular colorant, the resolution of the produced image is limited onlyby the electric field resolution produced by the electrode array 107,109. The molecular colorant additionally has virtually instantaneousswitching speed, beneficial to the needs of fast scanning (as describedwith respect to FIG. 3 hereinafter). In certain cases, the molecularcolorant may be contained in a polymeric layer. Polymers for producingsuch coatings are well-known, and include, for example, acrylates,urethanes, and the like. Alternatively, the colorant layer 101 may beself-assembled.

In one embodiment, the colorant layer 101 is offered as a substitute formatrix-addressed liquid crystal flat panel displays. As is well-knownfor such displays, each pixel is addressed through rows and columns offixed-position electrode arrays, e.g., 107, 109. The fixed-positionelectrode arrays 107, 109 consist of conventional crossbar electrodes111, 113 that sandwich the colorant layer 101 to form an overlappinggrid (matrix) of pixels, each pixel being addressed at the point ofelectrode overlap. The crossbar electrodes 111, 113 comprise parallel,spaced electrode lines arranged in electrode rows and columns, where therow and column electrodes are separated on opposing sides of thecolorant layer 101. Preferably, a first set of transparent crossbarelectrodes 107 (201, 203 in FIG. 2 described in detail hereinafter) isformed by thin film deposition of indium tin oxide (ITO) on atransparent substrate (e.g., glass). These row addressable pixelcrossbar electrodes 107 are formed in the ITO layer using conventionalthin film patterning and etching techniques. The colorant layer 101 andbackground substrate 103 are sequentially coated over or mounted to thetransparent electrode layer, using conventional thin film techniques(e.g., vapor deposition) or thick film techniques (e.g., silkscreen,spin cast, or the like). Additional coating techniques includeLangmuir-Blodgett deposition and self-assembled monolayers. Columnaddressable pixel crossbar electrodes 109 (202, 204 in FIG. 2) arepreferably constructed in like manner to the row electrodes 107. Thecolumn addressable pixel crossbar electrodes 109 may optionally beconstructed on a separate substrate that is subsequently adhered to thewhite coating using conventional techniques.

This display 100, 200 provides print-on-paper-like contrast, color,viewing angle, and omni-ambient illumination viewability by eliminationof the polarization layers required for known liquid crystal colorants.Using the described-display also allows a significant reduction in powerdrain. Whereas liquid crystals require a holding field even for a staticimage, the present molecules of the colorant layer 101 can be modal inthe absence of a current when bi-stable molecules are used. Thus, thepresent bi-stable colorant layer 101 only requires an applied voltagewhen a pixel is changed (the oxidation or reduction process) and onlyfor that pixel. The power and image quality improvements will providesignificant benefit in battery life and display readability, under awider range of viewing and illumination conditions for appliances (e.g.,wristwatches, calculators, cell phones, or other mobile electronicapplications) television monitors and computer displays. Furthermore,the colorant layer may comprise a mosaic of colored pixels using anarray of bi-stable color molecules of various colors for lowerresolution color displays.

Since each colorant molecule in colorant layer 101 is transparentoutside of the colorant absorption band, then multiple colorant layersmay be superimposed and separately addressed to produce higherresolution color displays than currently available. FIG. 2 is aschematic illustration of this second embodiment. A high resolution,full color, matrix addressable, display screen 200 comprises alternatinglayers of transparent electrodes—row electrodes 201, 203 and columnelectrodes 202 and 204—and a plurality of colorant layers 205, 207, 209,each having a different color molecule array. Since each pixel in eachcolorant layer may be colored or transparent, the color of a given pixelmay be made from any one or a combination of the color layers (e.g.,cyan, magenta, yellow, black) at the fall address resolution of thedisplay. When all colorant layers 205, 207, 209 for a pixel are madetransparent, then the pixel shows the background substrate 103 (e.g.,white). Such a display offers the benefit of three or more timesresolution over present matrix LCD devices having the same pixel densitybut that rely on single layer mosaic color. Details of the fabricationof the display are set forth in the above-mentioned co-pendingapplication.

The color to be set for each pixel is addressed by applying a voltageacross the electrodes directly adjacent to the selected color layer. Forexample, assuming yellow is the uppermost colorant layer 205, magenta isthe next colorant layer 207, and cyan is the third colorant layer 209,then pixels in the yellow layer are addressed through row electrodes 201and column electrodes 202, magenta through column electrodes 202 and rowelectrodes 203, and cyan through row electrodes 203 and columnelectrodes 204. This simple common electrode addressing scheme is madepossible because each colorant molecule is color stable in the absenceof an applied voltage.

FIG. 3 depicts a third embodiment, which employs scan-addressing ratherthan matrix-addressing. Matrix address displays are presently limited inresolution by the number of address lines and spaces that may bepatterned over the relatively large two-dimensional surface of adisplay, each line connecting pixel row or column to the outer edge ofthe display area. In this third embodiment, the bi-stable molecularcolorant layer 101 and background substrate 103 layer construction iscombined with a scanning electrode array printhead to provide a scanningelectrode display apparatus 300 having the same readability benefits asthe first two embodiments described above, with the addition ofcommercial publishing resolution. Scanning electrode arrays and driveelectronics are common to electrostatic printers and their constructionsand interfaces are well-known. Basically, remembering that the bi-stablemolecular switch does not require a holding voltage, the scanningelectrode array display apparatus 300 changes a displayed image byprinting a pixel row at a time. The scanning electrode array displayapparatus 300 thus provides far greater resolution by virtue of theability to alternate odd and even electrode address lines along opposingsides of the array, to include multiple address layers with pass-througharray connections and to stagger multiple arrays that proportionatelysuperimpose during a scan. The colorant layer 101 may again be patternedwith a color mosaic to produce an exceptionally high resolution scanningcolor display.

More specifically, the third embodiment as shown in FIG. 3 comprises adisplay screen 302, a scanned electrode array 304, and array translationmechanism 301 to accurately move the electrode array across the surfaceof the screen. The display screen 302 again comprises a backgroundsubstrate 103, a transparent view-through layer 105, and at least onebi-stable molecule colorant layer 101. The colorant layer 101 mayinclude a homogeneous monochrome colorant (e.g., black) or color mosaic,as described herein above. The scanned electrode array 304 comprises alinear array or equivalent staggered array of electrodes in contact ornear contact with the background substrate 103. A staggered array ofelectrodes may be used, for example, to minimize field crosstalk betweenotherwise adjacent electrodes and to increase display resolution.

In operation, each electrode is sized, positioned, and electricallyaddressed to provide an appropriate current, represented by the arrowlabeled “I”, across the colorant layer 101 at a given pixel locationalong a pixel column. The current I is oriented perpendicular to theplane of the colorant layer 101 by placing a common electrode (e.g., anITO layer) on the opposing coating side to the electrode array. Theamount of charge that flows through the pixel, and thus the number ofmolecules that is oxidized or reduced and thus the optical density ofthe pixel, is determined by the amount of time the electrode spends atthe pixel location. Additional information regarding alternateembodiments and scanning mechanisms are discussed in the above-mentionedco-pending application.

Present Embodiments

In accordance with the embodiments disclosed herein, a thin-filmelectrochemical cell is made into an optical switch that can alternateamong various color states by charging or discharging theelectrochemical cell to various potentials. This mechanism is completelydifferent from any previously described electrochromic or chromogenicmaterial. The general idea is to create a thin film structure thatcontains (a) a layer of a material that can easily be oxidized, (b) anintermediate solid or highly viscous electrolyte, and (c) a layer of amaterial with a substituted tetrazole that can be easily reduced. In theprocess of the oxidation or reduction, either or both materials (a) and(c) should undergo a strong color change. This three-layer system issandwiched between two electrodes, at least one of which may be any typeof transparent conductor. Under the influence of an applied electricpotential, electrons are removed from the easily oxidized material andtransferred through external circuitry to the material that can bereduced. Since one of these materials undergoes a color change, theresult is a switch in the color state of the device.

However, both the oxidized and reduced materials can be reconverted backto their original redox state (or color states) by applying a reversedelectric potential (which is in some cases achieved simply by shortingthe electrodes).

Substituted tetrazoles are preferably employed in the practice of theembodiments disclosed herein. Tetrazole itself is represented by theformula

while substituted tetrazoles are represented by the formula

Where R₁, R₂, R₃, and R₄ are independently H, alkyls, or aryls.Preferably, two of the substituents can be alkyls and/or aryls, and therest hydrogen. Examples of suitable aryls include phenyl, naphthyl, andanthracyl. Further, the ring carbon may be in the 3-position, as shownabove, or in the 2-position.

One molecular example for this model, employing oxidation/reduction viacurrent flow, is illustrated as follows:

The foregoing molecular system is bi-stable, being electrochemicallyswitchable between a purple color and colorless. The following molecularsystem is tri-stable, being electrochemically switchable between blue,magenta, and colorless.

The HOMO-LUMO (Highest Occupied Molecular Orbital—Lowest UnoccupiedMolecular Orbital) gap of the chromogenic materials can be tuned bysubstituting various chemical groups onto the molecules. Thus, themolecules are switchable between two (or more) colors or from one colorto a transparent state, and at the same time the transmissivity and/orreflectivity of the multi-layer system (electrodes plus molecularlayers) switches between two desirable states.

FIGS. 4a-4 b depict one embodiment of the color-switching device 20 ofthe present invention. The device 20 comprises three layers 22, 24, 26sandwiched between two electrodes 28, 30. Layer 22 is an electrolyte,sandwiched between reductant layer 24 and oxidant layer 26.

For the layers 22, 24, 26 shown in FIGS. 4a-4 b, sufficiently thickmolecular films are grown, for example using Langmuir-Blodgetttechniques, vapor phase deposition, or electrochemical deposition, suchthat appropriate thicknesses of each layer have been deposited. Anotherdeposition technique is to suspend the molecules as a monomer/oligomeror solvent-based solution that is thick film coated (e.g., reverse roll)or even spin-coated onto the substrate. At least one of the twoelectrodes 28, 30 should be a transparent conductor, such as indium-tinoxide.

The cell 20 is operated as a thin-film rechargeable battery, with anexternal potential 32 applied to drive the half reactions of the twoactive layers 24, 26 in the desired direction. The electrolyte layer 22can be one of the advanced electrolytes currently used in batterytechnology, In this case, it is not the diffusion of ions into amaterial that controls the speed and intensity of a color change, butrather the flow of electrons e along path 34, and the ion conductivitythrough the solid electrolyte 22 simply acts to maintain chargeneutrality. When the reverse color change is desired, the two electrodes28, 30 may be shorted to one another to discharge the battery 20. Thiscurrent may be harnessed to provide useful work in some other part ofthe system, thus leading to a more energy efficient color-switchingsystem.

FIGS. 4a and 4 b are before and after representations of a process inwhich layer 24 is reduced, as represented by layer 24′ in FIG. 4b, andlayer 26 is oxidized, as represented by layer 26′ in FIG. 4b. FIG. 4aalso shows the direction of flow of electrons 34 in the cell 20 to carryout the switching (this may be accomplished by attaching an externaldrive voltage, or in the case of a “charged battery”, simply shortingthe electrodes 28, 30 together. In this example, both the layers 24, 26are transparent before the oxidation/reduction process (FIG. 4a). Afterthe current flows through the cell 20, at least some of layer 24 isreduced and at least some of layer 26 is oxidized. In this example, thematerial in layer 26 was designed to change color, as shown by thestippling of layer 26′ in FIG. 4b.

The cell 20 is a magnified version of a single pixel in FIGS. 1-3. Inother words, the upper electrode 30 is a portion of one of the crossedwires or ribbons 107, and the bottom electrode 28 is the perpendicularcrossed wire 109 for a particular pixel.

INDUSTRIAL APPLICABILITY

The molecular structures disclosed herein are expected to find use inoptical switch and display applications.

What is claimed is:
 1. An electrochemically-activated optical switchcomprising a molecular system configured between a pair of electrodes,said molecular system including at least one organic non-polymericmolecule that changes color when oxidized or reduced by an electriccurrent.
 2. The optical switch of claim 1 wherein said organicnon-polymeric molecule comprises a substituted tetrazole.
 3. The opticalswitch of claim 2 wherein said substituted tetrazole is represented bythe formula

where R₁, R₂, R₃, and R₄ are independently H, alkyls, or aryls and thering carbon is in the 3-position.
 4. The optical switch of claim 3wherein any two of R₁, R₂, R₃, and R₄ are alkyls and/or aryls, and theremainder hydrogen.
 5. The optical switch of claim 3 wherein the ringcarbon is in the 2-position.
 6. The optical switch of claim 3 whereinsaid molecular system comprises:

wherein (I) is purple and has a ΔE_(HOMO/LUMO)=2 eV and wherein (II) iscolorless and has a ΔE_(HOMO/LUMO)>3.5 eV.
 7. The optical switch ofclaim 3 wherein said molecular system comprises:

wherein (III) is blue and as a ΔE_(HOMO/LUMO)=1.85 eV, wherein (IV) ismagenta and has a ΔE_(HOMO/LUMO)=2.35 eV, and wherein (V) is colorlessand has a ΔE_(HOMO/LUMO)>3.5 eV.